FIG. 1A is a block diagram of an exemplary prior art Off Highway Vehicle. In particular, FIG. 1A generally reflects atypical prior art diesel-electric Off Highway Vehicle. Off Highway Vehicles include locomotives and mining trucks and excavators, where mining trucks and excavators range from 100-ton capacity to 400-ton capacity, but may be smaller or larger. Off Highway Vehicles typically have a power weight ratio of less than 10 h.p. per ton with a ratio of 5 h.p. per ton being common. Off Highway Vehicles typically also utilize dynamic or electric braking. This is in contrast to a vehicle such as a passenger bus that has a ratio of 15 h.p. per ton or more and utilizes mechanical or resistive braking.
As illustrated in FIG. 1A, the Off Highway Vehicle 100 includes a diesel primary power source 102 driving an alternator/rectifier 104. As is generally understood in the art, the alternator/rectifier 104 provides DC electric power to an inverter 106 that converts the AC electric power to a form suitable for use by a traction motor 108. One common Off Highway Vehicle configuration includes one inverter/traction motor per wheel 109, with two wheels 109 comprising the equivalent of an axle (not shown). Such a configuration results in one or two inverters per Off Highway Vehicle. FIG. 1A illustrates a single inverter 106 and a single traction motor 108 for convenience. By way of example, large excavation dump trucks may employ motorized wheels such as the GEB23™ AC motorized wheel employing the GE150AC™ drive system (both of which are available from the assignee of the present system).
Strictly speaking, an inverter converts DC power to AC power. A rectifier converts AC power to DC power. The term “converter” is also sometimes used to refer to inverters and rectifiers. The electrical power supplied in this manner may be referred to as prime mover power (or primary electric power) and the alternator/rectifier 104 may be referred to as a source of prime mover power. In a typical AC diesel-electric Off Highway Vehicle application, the AC electric power from the alternator is first rectified (converted to DC). The rectified AC is thereafter inverted (e.g., using power electronics such as Insulated Gate Bipolar Transistors (IGBTs) or thyristors operating as pulse width modulators) to provide a suitable form of AC power for the respective traction motor 108.
As is understood in the art, traction motors 108 provide the tractive power to move Off Highway Vehicle 100 and any other vehicles, such as load vehicles, attached to Off Highway Vehicle 100. Such traction motors 108 may be an AC or DC electric motors. When using DC traction motors, the output of the alternator is typically rectified to provide appropriate DC power. When using AC traction motors, the alternator output is typically rectified to DC and thereafter inverted to three-phase AC before being supplied to traction motors 108.
The traction motors 108 also provide a bracing force for controlling speed or for slowing Off Highway Vehicle 100. This is commonly referred to as dynamic braking, and is generally understood in the art. Simply stated, when a traction motor 108 is not needed to provide motivating force, it can be reconfigured (via power switching devices) so that the motor operates as an electric power generator. So configured, tie traction motor 108 generates electric energy which has the effect of slowing the Off Highway Vehicle. In prior art Off Highway Vehicles, such as illustrated in FIG. 1A, the energy generated in the dynamic braking mode is typically transferred to resistance grids 110 mounted on the vehicle housing. Thus, the dynamic braking energy is converted to heat and dissipated from the system. Such electric energy generated in the dynamic braking mode is typically wasted.
It should be noted that, in a typical prior art DC hybrid vehicle, the dynamic braking grids 110 are connected to the traction motors 108. In a typical prior art AC hybrid vehicle, however, the dynamic braking grids are connected to the DC traction bus 122 because each traction motor 108 is normally connected to the bus by way of an associated inverter 106 (see FIG. 1B). FIG. 1A generally illustrates an AC hybrid vehicle with a plurality of traction motors; a single inverter is depicted for convenience.
FIG. 1B is an electrical schematic of a typical prior art Off Highway Vehicle 100. It is generally known in the art to employ a single electrical energy source 102, however, two or more electrical energy sources may be employed. In the case of a single electrical energy source, a diesel engine 102 coupled to an alternator 104 provides the primary source power 104. In the case where two or more electrical energy sources 102 are provided, a first system comprises the prime mover power system that provides power to the traction motors 108. A second system (not shown) provides power for so-called auxiliary electrical systems (or simply auxiliaries). Such an auxiliary system may be derived as an output of the alternator, from the DC output, or from a separate alternator driven by the primary power source. For example, in FIG. 1B, a diesel engine 102 drives the prime mover power source 104 (e.g., an alternator and rectifier), as well as any auxiliary alternators (not illustrated) used to power various auxiliary electrical subsystems such as, for example, lighting, air conditioning/heating, blower drives, radiator fan drives, control battery chargers, field exciters, power steering, pumps, and the like. The auxiliary power system may also receive power from a separate axle driven generator. Auxiliary power may also be derived from the traction alternator of prime mover power source 104.
The output of prime mover power source 104 is connected to a DC bus 122 that supplies DC power to the traction motor subsystems 124A-124B. The DC bus 122 may also be referred to as a traction bus 122 because it carries the power used by the traction motor subsystems. As explained above, a typical prior art diesel-electric Off Highway Vehicle includes two traction motors 108, one per each wheel 109, wherein the two wheels 109 operate as an axle assembly, or axle-equivalent. However, a system may be also be configured to include a single traction motor per axle or configured to include four traction motors, one per each wheel 109 of a two axle-equivalent four-wheel vehicle. In FIG. 1B, each traction motor subsystem 124A and 124B comprises an inverter (e.g., inverter 106A and 106B) and a corresponding traction motor (e.g., traction motor 108A and 108B, respectively).
During braking, the power generated by the traction motors 108 is dissipated through a dynamic braking grid subsystem 110. As illustrated in FIG. 1B, a typical prior art dynamic braking grid subsystem 110 includes a plurality of contactors (e.g., DB1-DB5) for switching a plurality of power resistive elements between the positive and negative rails of the DC bus 122. Each vertical grouping of resistors may be referred to as a string. One or more power grid cooling blowers (e.g., BL1 and BL2) are normally used to remove heat generated in a string due to dynamic braking. It is also understood that these contactors (DB1-DB5) can be replaced by solid-state switches like GTO/IGBTs and can be modulated (like a chopper) to control the effective dynamic brake resistance.
As indicated above, prior art Off Highway Vehicles typically waste the energy generated from dynamic braking. Attempts to make productive use of such energy have been unsatisfactory. For example, one system attempts to use energy generated by a traction motor 108 in connection with an electrolysis cell to generate hydrogen gas as a supplemental fuel source. Among the disadvantages of such a system are the safe storage of the hydrogen gas and the need to carry water for the electrolysis process. Still other prior art systems fail to recapture the dynamic braking energy at all, but rather selectively engage a special generator that operates when the associated vehicle travels downhill. One of the reasons such a system is unsatisfactory is because it fails to recapture existing braking energy and fails to make the captured energy available for reuse on board the Off Highway Vehicle.
Therefore, there is a need for an energy management system and method that control when energy is captured and stored, and when such energy is regenerated for later use.